Electron multiplier having an inclined field



Feb. 15, 1966 QODRICH ETAL 3,235,765

ELECTRON MULTIPLIER HAVING AN INCLINED FIELD Filed April 13, 1962 2 Sheets-Sheet 1 SOURCE INVENTggiH RGE w. 600 I F I g 4 BY DEN M. SMITH ATTORNEY Feb. 15, 1966 G. w. GOODRICH ET AL 3,235,765

ELECTRON MULTIPLIER HAVING AN INCLINED FIELD Filed April 15, 1962 2 Sheets-Sheet 2 INCOMING PARTICLES INVE'NTORS 92 BY GEORGE W. GOODRICH 6 HAYDEN M. SMITH ATTOR-NE United States Patent Ofi Fice 3,235,765 Patented Feb. 15, 1966 3,235,765 ELECTRON MULTIPLIER HAVING AN INCLINED FIELD George W. Goodrich, Oak Park, and Hayden M. Smith,

Whitmore Lake, Mich., assignors to The Bendix Corporation, Southfield, Mich., a corporation of Delaware Filed Apr. 13, 1962, Ser. No. 187,384 Claims. (Cl. 315-12) This invention pertains to an electron multiplier wherein electrons are accelerated repeatedly into a secondary emissive surface .by an electric field inclined to the longitudinal dimension of the surface.

Electron multipliers of the art include the type Where electrons are caused to cycloid and form corresponding multiplication stages along a secondary emissive resistive surface which has both an electric and a magnetic field applied thereto. More recently, multipliers have been constructed of small tubes or channels using a continuous secondary emissive material and an electrical field parallel to the longitudinal dimension of the multiplier with the multiplication stages, or electron collisions with the secondary emissive surface, being due to the transverse component of velocity of the entering particles and emission velocity of the secondary particles. Such multipliers are referred to as continuous channel multipliers.

This invention improves the prior devices by utilizing just an electric field, inclined to a secondary emissive surface to accelerate the electrons into the surface. In other words, the electric field has a component directed into the surface. The degree of multiplication can be controlled and no magnetic fields are needed. Secondary electrons formed by an electron impact are lifted against the component of the electric field directed into the surface by their energy of emission and the field then overcomes the energy of emission and accelerates them again into the surface to form a multiplication. The distance an electron travels before striking the surface can be controlled :by controlling the strength of the electric field and its angle to the surface. The greater the distance an electron travels, the greater the energy it will accumulate before striking "the surface again. In this invention, the distance an electron travels before striking the secondary emissive surface can be controlled and this feature is used in .one embodiment to reduce noise.

It is, therefore, an object of this invention to provide an electron multiplier wherein the only field is an electric field and which is capable :of relatively accurate control of the multiplication over a given length of multiplier surface.

It is an object of this invention to provide an electron multiplier having a secondary emissive resistive surface with a voltage source placed across the longitudinal dimension of the surface to cause current flow in and potential drop across the longitudinal dimension of the surface, and a second surface spaced from and cooperating with the :secondary emissive resistive surface to create an electric field which is transverse to the emissive resistive surface so that electrons will be accelerated into the surface of the secondary emissive material.

It is a further object of this invention to make the electric field less transverse at the input end of the multiplier than at the output end so that the electrons travel farther and gain more energy before contacting the secondary emissive surface at the input end of the multiplier than at the output end. This reduces noise because noise is approximately equal to the square root of the multiplied particles at a single impact and the impacts in the earlier or input portions of the multiplier are of greater force, so that the noise is a lower percentage of total amplification. Further, there is more likelihood of averaging noise effects in the latter stages of amplification. This will be illustrated subsequently by a specific example.

It is an object of this invention to provide a continuous channel multiplier with end faces which are not normal to the long axis of the multiplier exposing a secondary emissive surface, thereby facilitating the efficient introduction of primary particles by directing the primary particles into the exposed secondary emissive surface.

These and other objects will become more apparent when preferred embodiments of this invention are considered in connection with the drawings in which:

FIGURE 1 is a view in perspective of a preferred embodiment of this invention;

FIGURE 2 is asection taken along 2-2 of FIGURE 1;

FIGURE 3 is an array of multipliers, such as shown in FIG'URES 1 and 2, in combination with a photocathode and phosphor screen;

FIGURE 4 is a sectional schematic view of a further embodiment of this invention having non-parallel ends;

FIGURE 5 is a sectioned schematic view of another embodiment of this invention; and,

FIGURE 6 is a sectioned view of parallel plates which provide a secondary emissive surface and supply the transverse electric field.

In FIGURES 1 and -2 is seen an evacuated housing 19 in which is positioned a glass or insulative tube 20'which has its receiving or input end 22 and emitting or output end 24 cut at an angle to the longitudinal axis 26 of the tube so that ends 22 and 24 are parallel. A uniform secondary emissive resistive coating 28, such as tin oxide, is formed according to methods well-known to the art, on the inside of tube 20. Conductive coatings are placed on ends 22 and 24 so that when a potential supplied by battery 30 is applied to any portion of either end, the entire periphery of that end will be at that potential.

The resistive coating 28 makes contact with the conductive coatings '22 and 24. A typical potential for end 22 is a minus 2,000 volts while end 24 is at 0 volt. The tube is preferably aligned :at an angle to incoming particles from a source 34, and a collecting anode 36 is placed at the output end of tube 20 and is at a potential slightly higher than the output end to attract electrons coming therefrom.

Since the ends 22 and 24 are cut at an angle to tube 20, it will be apparent to those skilled in the art that lines of equi-potential 40 will be parallel to the ends 22, 24 and established continuously along resistive coating 28 which has a uniform potential drop along every longitudinal line thereof. However, as the length to the diameter ratio of tube 20 increases, the lines of equi-potential near the center of the tube will tend to become more nearly perpendicular to the walls of tube 20, and therefore, the following discussions will pertain primarily to the ends of the tube. For illustration purposes, however, it will be assumed that the tubes are sufficiently short so that the lines of equi-potential do not tend to become :more perpendicular to the walls near the center of the tube.

The electric field lines are perpendicular to the lines of equip'otential 40. Particles from source 34 would be accelerated along the field lines and strike the surface 28, as indicated in FIGURE 2, causing secondary emission. The

energy of secondary emission would cause the multiplied electrons to be released from the surface in random directions from the surface but these particles would be influenced by the electric field and accelerated again into the surface 28 to cause further secondary emission. The paths of the secondary electrons would depend upon the direction of an energy with which the particles or primary electrons strike the surface, the strength and direction of the electric field, and the nature of the secondary emissive material. The longer the path of an electron, the more energy it gathers before it strikes the surface and hence the more electrons are emitted, but the shorter this path, the more times the electrons strike the surface in a given tube length and hence the greater the multiplication. Therefore, there is an optimum of the strength and direction of the electric field which will provide electron paths that yield the greatest electron multiplication and varying either of these factors will lower the electron multiplication a corresponding amount. Electron multiplication, with a relatively large degree of control can be accomplished by utilizing only an electric field which, as has been shown, is transverse to the multiplying surface.

By combining a large number of multiplier tubes 20, FIGURES 1 and 2, in an evacuated envelope 41, into an array 42, as shown schematically in FIGURE 3, an image intensifier is provided. A photocathode 44 receives an image and emits electrons corresponding to the image intensity. These electrons are directed towards the array 42 and multiplied by the tubes in array 42 and the multiplied electrons then are emitted from array 42 and impinge upon phosphor screen 46. Screen 46 receives electrons on one side and emits light of corresponding intensity on the other, forming an image similar to that received by photocathode 44, but much intensified. This arrangement has a basic advantage over intensifiers in which the tubes or channels have square ends in that the initial impact of primary photo electrons with the tubes occurs more consistently close to the input end of the channels resulting in higher and more consistent multiplication.

The array 42 may be made by many methods known to the art. If the array were made entirely of a homogenous secondary emitting material, the efiect previously mentioned of the equipotential lines becoming more nearly perpendicular to the side walls at the tubes center would be minimized.

A further embodiment of this invention is shown in FIGURE 4. In this embodiment the lines of equi-potential 60 in the channel or tube are not parallel, but rather are varied in a manner to reduce noise in the multiplier. As in the previous embodiments, an insulative tube or channel 50 is placed in an evacuated housing 51 and is coated with a secondary emissive resistive coating such as tin oxide 52 to a thickness which will give a desired current flow when a potential from a battery 54 is placed across the ends thereof. As in the previous embodiment the input or receiving end 56 and the output or emitting end 58 is coated with a conductive material such as gold or silver paint, and the leads from battery 54 are connected to the conductive coating. However, in this embodiment, ends 56 and 58 are not parallel to one another, but rather end 56 is more nearly perpendicular to the longitudinal axis of tube 50 than is end 58 so that the lines of equipotential 60 are more nearly perpendicular to the surface and gradually become more angular as the output end 58 is reached. A source 62 is aligned with the input end 56 and preferably emits particles along a line such as line 64 so that they are directed into the secondary emissive resistive surface 52. A collecting anode 66 is aligned at the output end and is placed at a slightly higher potential than the output end to attract electrons coming therefrom.

The electric field is perpendicular to the lines of equipotential and the field near the input end 56 is nearly parallel to the surfaces 52 so that the secondary electrons which are released from surface 52 upon an impact from an incoming or primary electron, travel a longer distance before again striking the secondary emissive surface. Since these electrons are being accelerated at a constant acceleration, the longer their path, the more time they are being accelerated and the more force with which they hit the secondary emissive surface. This means that more electrons are released per impact near the input end of the channel 50 than are released near the output end 56 wherein the electron trajectory is shown to become smaller due to the fact that the field lines become more nearly perpendicular to the surface and the 4 electrons released are more quickly directed into the surface. Even though the electron trajectory is smaller, the total multiplication near the output end is greater than that near the input end since more impacts are made per inch of tube length.

Noise is generated in a multiplier of the kind shown in FIGURES 1 and 4 due to the fact that for any particular impact the resulting secondary electrons are emitted in a statistical manner and vary from the average number of secondary electrons released. This variation or noise, generally speaking, is in a square root proportion to the number of electrons released per impact. It is preferable to have the high energy impacts near the input end of the tube, as they are in the embodiment of FIG- URE 4, because the noise is lower percentagewise and noise generated at the input is more harmful than noise generated near the output.

This may be illustrated by an extremely simplified example. Assume that 9 secondary electrons are released per impact near the input end. The noise electrons would be plus or minus 3 (square root of nine) and are :33 per-cent of the multiplication. Assume that near the output end, only 4 electrons are released per impact, the noise electrons would be plus or minus 2 (the square root of 4) which is :50 percent of the multiplication. There are so many more electrons impacting near the output end that there are near the input end, so that deviations near the output end have a greater statistical tendency to average out, whereas deviations near the input end do not have as much opportunity to average out, making them more harmful. In the embodiment of FIGURE 4, the noise electrons are a smaller percent of the total electrons, thereby, improving overall multiplier accuracy.

In FIGURE 5 is shown an embodiment using in an evacuated enclosure 69, a tube or channel 70 with a secondary emissive surface 72 formed on a portion of, but not all of, the interior surface thereof. A source 73 is at the input end of the tube 70 and an anode 75 is at the output end. Tube 70 has end faces which are perpendicular, in this embodiment, to the longitudinal axis of the tube. A conductive coating 74 is formed on the input or receiving end of the tube having a portion 76 extending into the tube which establishes the geometry of the equipotential lines near the input end, and the conductive coating 78 on the output or emitting portion of the tube extending a distance 80 into the interior of the tube which establishes the geometry of the equipotential lines near the output end. The potential supplied by battery 82 is applied to the ends of tube 70 but the equipotential lines 81 are not parallel to the ends of tube 70 but are determined by the lengths of coatings 76 and 80. In this embodiment, coating 76 does not extend as far into tube 70 as does coating 80 sothat the effect of the embodiment of FIGURE 4 is obtained.

Therefore, by using a standard tube, field configurations like those in the embodiments of FIGURE 4 or FIGURE 1, may be achieved by simply forming the conductive coatings 76 and 80 inwardly at predetermined distances. Regardless of the angle of the tube end faces with the longitudinal axis of the tube, any desired angle between the lines of equipotential and the axis may be effected. This permits greater freedom in designing the electron optics of the input section.

A further embodiment illustrating the application of this invention is shown in FIGURE 6 where two parallel plates, instead of a tube or channel, are used. Upper plate and lower plate 92 are in evacuated housing 93 and are made of insulative material such as glass or plastic which forms support blocks for glass strips 94, 96 respectively. Glass strips 94, 96 are coated with a secondary emissive resistive material as described for the embodiment of FIGURES 1 and 2 and strips 94, 96 are placed as shown with lower strip 96 extending leftwardly a greater degree than strip 94 so that when a potential, such as -2,000 volts, from source 98 is applied to the left ends of strips 94 and 96 and a potential such as 0 volt is applied to the right end or output end of strips 94, 96 equipotential lines 100 will be formed at an angle to the longitudinal dimension. Anode or collector 102 is placed in slots 104 and 106' in Kel-F support blocks 90, 92 respectively. The operation of this embodiment is similar to that of the previous embodiments with the incoming particles being directed into the secondary emissive coating on strip 96 and electron multiplication results from electrons being repeatedly accelerated into the strip 96. By varying the potential at the ends of strips 94, 9'6, field configurations for low noise or other purposes may be realized without changing the configuration of the multiplier.

Milliammeters have been shown in each of the anode circuits but other measuring means, control means, or indicating means may be placed therein.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

Having thus described our invention, we claim:

1. A multiplier comprising wall means being provided with secondary emissive material and defining at least a partially enclosed substantially rectilinear multiplying path having a longitudinal dimension substantially larger than its lateral dimension,

said multiplying path being totally clear of field producing physical obstructions independent of said wall means,

said wall means at one end of its longitudinal dimension being adapted to receive particles to be multiplied,

collecting means being positioned at the other end of the wall means longitudinal dimension to collect the multiplied particles,

means for establishing a potential gradient at a majority of increments along said longitudinal dimension of said wall means from said one end to said other end,

means for establishing a substantially total field which is at a substantial majority of points along said longitudinal dimension from said one end to said other end an electric field inclined to said wall means with the electric field lines being substantially parallel to one another,

envelope means to enclose said multiplying path.

2. The multiplier of claim 1 with,

emitting means for emitting particles to be multiplied positioned at said one end of the wall means longitudinal dimension,

said means for establishing a potential gradient effecting a potential gradient at substantially all increments along said longitudinal dimension of said wall means from said one end to said other end,

said means for establishing a field inclined to said wall means comprising means for placing at least two longitudinally and laterally spaced points on said wall means at the same potential so that the total field at substantially all points along said longitudinal dimension from said one end to said other end is an electric field inclined to said wall means, with the angle of inclination of said field being determined by the longitudinal spacing between said points.

3. The multiplier of claim 2 wherein said Wall means define two closely spaced flat parallel plates.

4. A multiplier comprising wall means being provided with secondary emissive material and defining at least a partially enclosed longitudinal dimension substantially larger than its lateral dimension,

said multiplying path being totally clear of field pro ducing physical obstructions independent of said wall means,

said wall means at one end of its longitudinal dimension being adapted to receive particles to be multiplied,

collecting means being positioned at the other end of the wall means longitudinal dimension to collect the multiplied particles,

means for establishing a potential gradient at a majority of increments along said longitudinal dimension of said wall means from said one end to said other end,

means for establishing a substantially total field which is at a substantial majority of points along said longitudinal dimension from said one end to said other end an electric field inclined to said wall means, envelope means to enclose said multiplying path, emitting means for emitting particles to be multiplied positioned at said one end of the wall means longitudinal dimension, said means for establishing a potential gradient effecting a potential gradient at substantially all increments along said longitudinal dimension of said wall means from said one end to said other end,

said means for establishing a field inclined to said wall means comprising means for placing at least two longitudinally and laterally spaced points on said wall means at the same potential sothat the total field at substantially all points along said longitudinal dimension from said one end to said other end is an electric field inclined to said wall means, with the angle of inclination of said field being determined by the longitudinal spacing between said points,

said secondary emissive surface is a continuous resistive surface from said emitting end to said collecting end,

means for establishing a potential gradient at substantially all increments along said longitudinal dimension of said wall means,

said means for establishing a potential gradient comprising a voltage source connected tosaid secondary emissive surface at said emitting end and connected to said secondary emissive surface at said collecting end, causing a current flow in said secondary emissive resistive surface,

the electric field angle being more nearly parallel with the longitudinal axis of said wall means at the emitting means end of said multiplier than at the collecting means end of said multiplier.

5. The multiplier of claim 4 with said wall means being tubular in shape, said potential gradient voltage source being connected to said secondary emissive resistive coating at said emitting means end in at least two points, one of which is spaced further from the end of the tubular wall means than the other,

said potential gradient voltage source being connected to said end at said collecting means in at least two points, one of which is spaced further from the end of said tubular wall means than the other.

References Cited by the Examiner UNITED STATES PATENTS 2,141,322 12/1938 Thompson 313104 X 2,210,034 8/1940 Keyston 315-12 2,841,729 7/1958 Wiley 3 l3-104 2,932,768 4/1960 Wiley 313l04 X 3,062,962 11/1962 McGee 250-213 3,128,408 4/1964 Goodrich et al. 313-103 substantially rectilinear multiplying path having a 7 GEORGE N. WESTBY, Primary Examiner. 

1. A MULTIPLIER COMPRISING WALL MEANS BEING PROVIDED WITH SECONDARY EMISSIVE MATERIAL AND DEFINING AT LEAST A PARTIALLY ENCLOSED SUBSTANTIALLY RECTILINEAR MULTIPLYING PATH HAVING A LONGITUDINAL DIMENSION SUBSTANTIALLY LARGER THAN ITS LATERAL DIMENSION, SAID MULTIPLYING PATH BEING TOTALLY CLEAR OF FIELD PRODUCING PHYSICAL OBSTRUCTIONS INDEPENDENT OF SAID WALL MEANS, SAID WALL MEANS AT ONE END OF ITS LONGITUDINAL DIMENSION BEING ADAPTED TO RECEIVE PARTICLES TO BE MULTIPLIED, COLLECTING MEANS BEING POSITIONED AT THE OTHER END OF THE WALL MEANS LONGITUDINAL DIMENSION TO COLLECT THE MULTIPLIED PARTICLES, MEANS FOR ESTABLISHING A POTENTIAL GRADIENT AT A MAJORITY OF INCREMENTS ALONG SAID LONGITUDINAL DIMENSION OF SAID WALL MEANS FROM SAID ONE END TO SAID OTHER END, MEANS FOR ESTABLISHING A SUBSTANTIALLY TOTAL FIELD WHICH IS AT A SUBSTANTIAL MAJORITY OF POINTS ALONG SAID LONGITUDINAL DIMENSION FROM SAID ONE END TO SAID OTHER END AN ELECTRIC FIELD INCLINED TO SAID WALL MEANS WITH THE ELECTRIC FIELD LINES BEING SUBSTANTIALLY PARALLEL TO ONE ANOTHER, ENVELOPE MEANS TO ENCLOSE SAID MULTIPLYING PATH. 